Application of Electrical Resistivity and Elt:ctromagnetic Prospecting Methods for Ground water Studies Around Dembi near Debrezeit
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Date
1998-06
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Addis Ababa University
Abstract
Combined electrical resistivity and electromagnetic prospecting methods were
carried out around Dembi area near Debrezeit town. The analysis of the results
obtained from both geophysical methods has provided useful information
regarding the electrical characteristics of the subsurface which is related with the
thickness and depth ranges of layers. The physical property distribution which is
obtained by the two methods is analyzed with particular significance for ground
water.
Twenty vertical electrical sounding (yES) measurements using Schulumberger
configuration along four parallel profiles were conducted. The field apparent
resistivity data is plotted against electrode separation and initial model
parameters for each sounding station is obtained using auxiliary point method
and two layer master curves. For quantitative evaluation of the resistivity
soundings, the field data was submitted to a curve matching computer program.
In this program the apparent resistivities obtained as a function of electrode
spacings is converted to true resistivities as function of depth.
As result of inversion, a f().LJr_.I~l'~_~!J.~()e l~<:;!~c._ ~ection is constructed for all
profiles in the studied area. The investigation has made it possible to establish
the layer stratification along all the profiles. Generally, this geophysical
investigation around Dembi area has indicated a four layer earth structure.
The first layer is described by a more or less uniform thin layer of alluvial
deposits. Underlying the top layer is a second layer characterized by a very low
apparent resistivity value which represented the bottom part of the soil section
with considerable moisture and clay content. The third geoelectric layer is made
up of a vast portion of weathered and fractured basalt section which is assumed
to be the most favorable site for the accumUlation of subsurface water. From
hydrogeological point of view, this layer satisfies the physical condition to be
favorable host for ground water. Underlying the third layer a section identified by
comparatively low apparent resistivity value is interpreted to be highly saturated
volcanic ash and clay.It was also possible to observe a generally decreasing apparent resistivity values
towards the Western direction along each profile over the investigated area. A
decreasing trend of apparent resistivity values were also observed due to the
conductive nature of the ground towards North.
The results obtained from electromagnetic interpretation also showed
anomalously high conductivity responses around the third and the fourth
profiles. An increasing tendency of conductivity was also observed towards the
Western direction coinciding with the result obtained from the resistivity survey.
The results of both geophysical methods has shown strong correlation in that, a
decrease in apparent resistivity values in the sounding survey is associated with
an increase in electrical conductivity obtained from electromagnetic survey
towards North and West of the studied area.
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Keywords
Application of Electrical Resistivity